Optimized flash based cache memory

ABSTRACT

Embodiments of the invention relate to throttling accesses to a flash memory device. The flash memory device is part of a storage system that includes the flash memory device and a second memory device. The throttling is performed by logic that is external to the flash memory device and includes calculating a throttling factor responsive to an estimated remaining lifespan of the flash memory device. It is determined whether the throttling factor exceeds a threshold. Data is written to the flash memory device in response to determining that the throttling factor does not exceed the threshold. Data is written to the second memory device in response to determining that the throttling factor exceeds the threshold.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/112,894, filed May 20, 2011, the contents of which is incorporated byreference herein in its entirety which. U.S. patent application Ser. No.13/112,894 is related to U.S. patent application Ser. No. 13/112,601,filed May 20, 2011, the contents of which are hereby incorporated byreference in its entirety.

BACKGROUND

The present invention relates generally to memory, and morespecifically, to using a flash device as a cache memory.

Flash memory devices are a type of non-volatile storage devices that canbe electrically erased and reprogrammed in large blocks. Flash memorydevices store information in an array of memory cells made fromfloating-gate transistors. Single-level cell (SLC) flash memory devicesstore one bit of information in each cell. Multi-level cell (MLC) flashmemory devices can store more than one bit per cell by choosing betweenmultiple levels of electrical charge to apply to the floating gates ofits cells. Flash memory devices have a limited lifetime because they canonly support a finite number of program-erase (P/E) cycles. Contemporarycommercially available flash memory devices are typically guaranteed tosupport a specified number of P/E cycles before the wear begins toimpact the reliability of the storage.

Using flash memory devices as an alternative or supplement to dynamicrandom access memory (DRAM) and/or to hard disk drives (HDDs) isbecoming increasingly attractive as the cost of flash memory decreasesand the performance of flash memory increases. From a cost perspective,flash memory is currently about ten times less expensive thanconventional DRAM. The lower cost of flash memory allows much largerdata sets to be stored without a corresponding increase in the cost ofthe memory. From a performance perspective, contemporary flash memoriescurrently have about two orders of magnitude higher random readthroughput than traditional mechanical drives. One downside to the useof flash memory devices for cache memory is that flash memory deviceshave a limited life and may not be ideal for storing data subject to alarge number of updates.

BRIEF SUMMARY

An embodiment is a method that includes throttling accesses to a flashmemory device. The flash memory device is part of a storage system thatincludes the flash memory device and a second memory device. Thethrottling is performed by logic that is external to the flash memorydevice and includes calculating a throttling factor responsive to anestimated remaining lifespan of the flash memory device. It isdetermined whether the throttling factor exceeds a threshold. Data iswritten to the flash memory device in response to determining that thethrottling factor does not exceed the threshold. Data is written to thesecond memory device in response to determining that the throttlingfactor exceeds the threshold.

Another embodiment is a computer program product that includes acomputer readable storage medium having computer readable program codeembodied therewith. The computer readable program code includes computerreadable program code that is configured for throttling accesses to aflash memory device. The flash memory device is part of a storage systemthat includes the flash memory device and a second memory device. Thethrottling is performed by logic that is external to the flash memorydevice. The throttling includes calculating a throttling factorresponsive to an estimated remaining lifespan of the flash memorydevice. It is determined whether the throttling factor exceeds athreshold. Data is written to the flash memory device in response todetermining that the throttling factor does not exceed the threshold.Data is written to the second memory device in response to determiningthat the throttling factor exceeds the threshold.

A further embodiment is a method that includes throttling accesses to aflash memory device. The flash memory device is part of a storage systemthat includes the flash memory device and a second memory device. Thethrottling is performed by logic that is external to the flash memorydevice. The throttling includes calculating a lifespan throttling factorresponsive to an estimated remaining lifespan of the flash memorydevice. In addition, a throughput throttling factor responsive to anestimated maximum throughput of the flash memory device is calculated,along with a maximum throttling factor responsive to the lifespanthrottling factor and the throughput throttling factor. It is determinedwhether the maximum throttling factor exceeds a threshold. In responseto determining that the maximum throttling factor exceeds the threshold,at least one of data size throttling, prefetch window size throttling,and usage throttling are performed. Data is written to at least one ofthe flash memory device and the second memory device in response to thethrottling.

A further embodiment is a computer program product that includes acomputer readable storage medium having computer readable program codeembodied therewith. The computer readable program code includes computerreadable program code that is configured for throttling accesses to aflash memory device. The flash memory device is part of a storage systemthat includes the flash memory device and a second memory device. Thethrottling is performed by logic that is external to the flash memorydevice. The throttling includes calculating a lifespan throttling factorresponsive to an estimated remaining lifespan of the flash memorydevice. A throughput throttling factor responsive to an estimatedmaximum throughput of the flash memory device is also calculated alongwith a maximum throttling factor responsive to the lifespan throttlingfactor and the throughput throttling factor. It is determined whetherthe maximum throttling factor exceeds a threshold. In response todetermining that the maximum throttling factor exceeds the threshold, atleast one of data size throttling, prefetch window size throttling, andusage throttling are performed. Data is written to at least one of theflash memory device and the second memory device in response to thethrottling.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 illustrates a block diagram of a system for providing flash cachememory in accordance with an embodiment;

FIG. 2 is process flow for throttling a rate at which data is written toa flash cache memory in accordance with an embodiment;

FIG. 3 is a process flow for throttling a rate at which data is writtento a flash cache memory based on a data size in accordance with anembodiment;

FIG. 4 is a process flow for throttling a rate at which data is writtento a flash cache memory based on a prefetch window size in accordancewith an embodiment; and

FIG. 5 is a process flow for throttling a rate at which data is writtento a flash cache memory based on a data usage in accordance with anembodiment.

DETAILED DESCRIPTION

An embodiment utilizes flash memory as a second level cache in between adynamic random access memory (DRAM) cache and hard disk drive (HDD)storage. Access to the flash cache memory is controlled and throttled bylogic located in a storage controller. Access to the flash cache memoryis throttled based on characteristics of the data that is being stored(e.g., whether the data accesses are sequential or random, the size ofthe data). In an embodiment, access to the flash memory cache iscompletely bypassed when certain data characteristics are detected. Bythrottling the data at the storage controller level based on datacharacteristics, a flash device may be used as a second level cachewhile still ensuring that a stated lifespan of the flash device will bepreserved.

In an embodiment, such as the one depicted in FIG. 1, a flash cachememory 106 is used as a second level cache in between a DRAM cachememory 104 and disk drives 108. Logic located in the storage controlleris utilized to determine where to store data (e.g., to the flash cachememory 106, the DRAM cache memory 104 and/or the disk drives 108) and totrack the location of data that was previously stored. The flash cachememory 106 can be used as a read cache and/or as a write cache. Whenused as a read cache, the flash cache memory 106 is populated with dataat the time data is returned from the disk drives 108 to the DRAM cachememory 104 and/or at a later time when the DRAM cache memory 104 evictsunmodified data. For modified data, the DRAM cache memory 104 (e.g.,being used as a write cache) has the option of de-staging the data intothe flash cache memory 106 rather than into the disk drives 108. Whenused in this fashion, in addition to absorbing over-writes, a flashcache memory 106 (e.g., being used as a write cache) accumulates a largenumber of writes, which can later be sorted and destaged to the diskdrives 108 in an efficient manner.

In the embodiment of the system shown in FIG. 1, the data flows throughthe DRAM cache memory 104 and use of the flash cache memory 106 isoptional. This allows logic in the storage controller 102 to throttlethe amount and type of data being sent to the flash cache memory 106.

A flash device is one example of a limited lifespan memory device thatmay be implemented by exemplary embodiments described herein. Otherlimited lifespan memory devices such as, but not limited to: phasechange memory (PCM) and other storage class memories (SCMs) may also beimplemented by exemplary embodiments.

Traditional DRAM based caching algorithms are primarily concerned withmaximizing the hit ratio (or minimizing the miss ratio) of the cache, soas to reduce the number of references to the underlying disk drives. Theleast recently used (LRU) algorithm, for example, exploits temporallocality in the workload to evict the oldest unreferenced data blocksfirst. The LRU algorithm has been successfully used reduce the number ofmisses for both reads and writes. Similarly, data pre-fetching isanother technique to improve the hit ratio for reads by exploitingspatial locality in the workload.

Focusing solely on hits and misses, however, may not be optimal forflash based caches. In particular, as opposed to DRAM where endurance isnot an issue, flash memory only allows a limited number of writes beforeit wears out and become unusable. Blindly focusing on hit and missmetrics, may therefore lead to excessive and uncontrolled wearing,making the lifespan of a flash cache short and unpredictable. Thisshortening of lifespan and unpredictability may be unacceptable for manytypes of implementations.

Multi-level cell (MLC) flash devices, for example, are currently ratedfor about five thousand wear cycles, and it would take only a few monthsto wear out such devices if used as a cache without any throttling. Tokeep servicing costs low, it would be desirable for a cache for anystorage system to have a lifespan of a few years and ideally guaranteedto achieve that lifespan irrespective of the workload.

One approach to tackle the above problem is to make the flash device runslower as it reaches the end of its lifespan. This extends the lifespanof the flash device but at the same time defeats its main purpose ofserving as a cache to improve the performance of the system. Thepresence of a slow flash device used as a cache may in fact, lower theoverall performance of the system. Embodiments described herein utilizetechniques that avoid situations where the flash cache becomes aperformance bottleneck while at the same time guaranteeing a minimumacceptable lifespan.

Embodiments described herein utilize throttling techniques that controlthe rate of inputs/outputs (I/Os) to the flash cache memory 106 in orderto remain below the throughput capacity of the flash device and thenetwork connecting the flash cache memory 106 to the DRAM cache memory104. This allows the flash device to maintain a minimum guaranteedlifespan. The throughput capacity of the flash device and the networkcan be determined empirically. This allows a flash control module (FCM)for the flash cache memory 106 (located, in an embodiment, in thestorage controller 102) to calculate, at any given time, the percentageof that capacity, p, currently being used. This percentage is used tocompute a throughput throttling factor (TTF), which is used by anembodiment to prevent the flash cache memory 106 from becoming aperformance bottleneck.

The remaining lifespan of the flash device can be determined in variousways. One option is to use the built-in wear indicator that somemanufactures provide for their flash devices. A simple wear indicator isa read-only counter that takes values in the range of one toone-hundred, where a value of one indicates no wearing at all, and avalue of one-hundred indicates that the device has reached its maximalnumber of writes. In the absence of such an indicator, the FCM cancalculate the wear by itself by keeping track of the writes to the flashdevice. In either case, the remaining lifespan of the flash device canbe computed. The disparity between the computed remaining lifespan, c,and the desired remaining lifespan, d, forms the basis for a secondthrottling factor used by an embodiment, referred to herein as alifespan throttling factor (LTF). The LTF is used to protect the flashdevice from reaching the end of its lifespan sooner than expected.

For simplicity, in an embodiment both throttling factors (TTF and LTF)take on values from −∞ to 1. For both factors, a negative valueindicates that no throttling is required and that data can be sentfreely to the flash cache memory 106. A value of 1 indicates that fullthrottling is required and that no data should be sent to the flashcache memory 106. A value between 0 and 1 indicates that some throttlingneeds to be applied to the flash cache memory 106, depending on thespecific value of the throttling factors.

In an embodiment, for the TTF, a threshold of 90% is chosen as thethroughput capacity above which throttling will be applied. This isdefined as follows: TTF=(p−90)/(100−90). Similarly, for the LTF athreshold of 10% above the desired remaining lifespan is chosen, and forvalues below that threshold throttling is applied. This is defined asfollows: LTF=(1.1d−c)/1.1d. Any threshold number can be implemented byan embodiment, and the thresholds may be programmable based onimplementation specific requirements. In another embodiment, thesethresholds are chosen to be more conservative for a write cache than fora read only cache, since in a read only cache, the data is protected bythe copy on the HDD, and the implementation can run closer to theend-of-life of the flash device without risking data loss. In anotherembodiment, where a mixed, read/write cache is implemented, as thehigher throttling factors are approached, flash cache memory 106 isdynamically split into a read and write portion, and faster wear out ispermitted in the read portion than in the write portion. This embodimenttakes advantage of the fact that media errors in the read portion willforetell the expected behavior in the write portion and allow advancenotice of end-of-life of the device.

FIG. 2 is an overall process flow for throttling a rate at which data iswritten to the flash cache memory 106 in accordance with an embodiment.In an embodiment, the process flow is implemented by logic in thestorage controller 102.

At block 202, the TTF, LTF and maximum (TTF, LTF) are calculated. Atblock 204, it is determined if the maximum of (TTF, LTF) is larger thana specified threshold. If the maximum (TTF, LTF) is not above thespecified threshold, then the process loops back to 204 becausethrottling is not required. If, it is determined at block 204 that themaximum (TTF, LTF) is above the specified threshold, then block 206 isperformed. At block 206, data size throttling is performed to limit theamount of data that is stored in the flash cache memory 106 based on thesize of the data and the maximum (TTF, LTF). An embodiment of a datasize throttling process is shown in FIG. 3. In an embodiment, the datasize throttling process at block 206 is performed for a specified numberof iterations and/or for a specified amount of time. Next, block 208 isperformed and TTF, LTF and maximum (TTF, LTF) are recalculated. At block210, it is determined if the data size throttling process is providingsufficient throttling by comparing the maximum (TTF, LTF) to a specifiedthreshold. If the maximum (TTF, LTF) is not above the specifiedthreshold, then processing continues at block 204.

If it is determined at block 210, that the maximum (TTF, LTF) is abovethe specified threshold, then processing continues at block 212 whereprefetch window size throttling is performed. An embodiment of aprefetch window size throttling process is shown in FIG. 4. In anembodiment, the prefetch window size throttling process at block 212 isperformed for a specified number of iterations and/or for a specifiedamount of time. Next, block 214 is performed and TTF, LTF and maximum(TTF, LTF) are recalculated. At block 216, it is determined if theprefetch window size throttling process is providing sufficientthrottling by comparing the maximum (TTF, LTF) to a specified threshold.If the maximum (TTF, LTF) is not above the specified threshold, thenprocessing continues at block 204. In another embodiment, processingcontinues at block 212 and the process loops back to 212 for a specifiednumber of iterations and/or for a specified amount of time beforelooping back up to block 204.

If it is determined at block 216, that the maximum (TTF, LTF) is abovethe specified threshold, then processing continues at block 218 whereusage throttling is performed. An embodiment of a usage throttlingprocess is shown in FIG. 5. Usage throttling handles the scenario inwhich the values of the TTF, or the LTF, or both, grow quickly andun-proportionally even under well behaved workloads. This may stem fromcertain physical characteristics of the flash device that cannot beanticipated in advance, and may lead to a scenario in which the flashcache memory 106 becomes an instant bottleneck and/or reaches itsend-of-life much sooner than expected. Under such conditions evensetting the flash prefetch window size to 0 (which means that only theoriginally requested data is sent to the flash cache memory 106) may notsuffice.

In an embodiment, the usage throttling process at block 218 is performedfor a specified number of iterations and/or for a specified amount oftime. Next, block 220 is performed and TTF, LTF and maximum (TTF, LTF)are recalculated. At block 222, it is determined if the usage throttlingprocess is providing sufficient throttling by comparing the maximum(TTF, LTF) to a specified threshold. If the maximum (TTF, LTF) is notabove the specified threshold, then processing continues at block 204.In another embodiment, processing continues at block 212 (or block 218)and the process loops back to 212 (or block 218) for a specified numberof iterations and/or for a specified amount of time before looping backup to block 204. If the maximum (TTF, LTF) is above the specifiedthreshold, then block 224 is performed and the use of the flash cachememory 106 is stopped. The system continues to use the DRAM cache memory104 along with its default caching algorithms communicating solely withthe disk drives 108 and bypassing the flash cache memory 106.

In an embodiment, after a specified amount of time and/or a specifiednumber of accesses, processing continues from block 224 to block 202 tosee if the flash cache memory 106 can be used for a current workload. Inan embodiment, the throttling processes shown in FIG. 2 are performed ina different order. In another embodiment, the maxfactor is compared todifferent values. In a further embodiment, a subset of the throttlingprocesses shown in FIG. 2 is performed. In a still further embodiment,additional or different throttling processes than those shown in FIG. 2are performed. For example, certain workloads or volumes could bepreferably throttled, or some hints from the applications could be usedin deciding what data is important to send to flash and what can bethrottled.

FIG. 3 is a process flow for throttling a rate at which data is writtento a flash cache based on a data size in accordance with an embodiment.In an embodiment, the process flow is implemented by logic in thestorage controller 102.

The process flow depicted in FIG. 3 avoids polluting the flash cachememory 106 with large blocks of data emanating from sequential reads orwrites. In an embodiment, these large blocks of data are sent directlyfrom the DRAM cache memory 104 to the disk drives 108. Not writing thesequential workloads to the flash cache memory 106 is advantageous forseveral reasons. First, sequential workloads have little locality andtherefore do not take advantage of the cache. In addition, thesequential performance of conventional disk drives is comparable to thatof flash devices and is unlikely to be improved through caching and mayeven worsen. Further, sequential workloads may cause much wear to flashdevices.

In the embodiment shown in FIG. 3, throttling is performed and a sizethreshold is set. Data sizes below the threshold are sent from the DRAMcache memory 104 to the flash cache memory 106, and data sizes above thethreshold are sent directly to the disk drives 108. The threshold can bestatic or dynamic or based on the throttling factors as in theembodiment shown in FIG. 3.

Referring to FIG. 3, at block 302, the maximum value of the TTF and theLTF is assigned to the variable “maxfactor.” In an embodiment, theprocess flow in FIG. 3 is called from the overall process flow depictedin FIG. 2 where the TTF and the LTF have been calculated. In anotherembodiment, part of the processing in block 302 includes calculating theTTF and the LTF. At block 304, it is determined if the maxfactor is lessthan 0. If the maxfactor is less than 0, processing continues at block306, where the maximum data size is set to infinity (i.e., all data issent to the flash cache memory 106). If it is determined at block 304,that the maxfactor is not less than zero, processing continues at block308 to determine if the maxfactor is less than 0.2. If the maxfactor isless than 0.2, processing continues at block 310, where the maximum datasize is set to 256 kilobytes (KB). Thus, only data that is less than 256KB in size is sent to the flash cache memory 106 from the DRAM cachememory 104, and data larger than 256 KB in size is sent from the DRAMcache memory 104 to the disk drives 108. If it is determined at block308, that the maxfactor is not less than 0.2, processing continues atblock 312 to determine if the maxfactor is less than 0.4.

Processing continues at block 314 if the maxfactor is less than 0.4 toprevent access to the flash cache memory 106 for data sizes over 128 KBand to block 316 if the maxfactor is not less than 0.4. At block 316, itis determined if the maxfactor is less than 0.6. Processing continues atblock 318 if the maxfactor is less than 0.6 to prevent access to theflash cache memory 106 for data sizes over 64 KB and to block 320 if themaxfactor is not less than 0.6. At block 320, it is determined if themaxfactor is less than 0.8. Processing continues at block 322 if themaxfactor is less than 0.8 to prevent access to the flash cache memory106 for data sizes over 16 KB and to block 324 if the maxfactor is notless than 0.8. At block 324, it is determined if the maxfactor is lessthan 0.95. Processing continues at block 326 if the maxfactor is lessthan 0.95 to prevent access to the flash cache memory 106 for data sizesover 4 KB and to block 328 if the maxfactor is not less than 0.95. Atblock 328, all data is prevented from being written to the flash cachememory. Thus, all memory is written directly from the DRAM cache memory104 to the disk drives 108.

Other embodiments where accesses to the flash cache memory 106 arethrottled based on data size have different maximum data size valuescorresponding to the same maxfactor values, different maxfactor valuescorresponding to different maximum data size values, fewer maxfactorvalues, or more max factor values. In another embodiment, the processingdepicted in FIG. 3 loops back up to block 302 after blocks 306, 310,314, 318, 322, 326 and 328 to recalculate the TTF, LTF and maxfactor foreach data access. In a further embodiment, the TTF, LTF and maxfactorare recalculated on a periodic basis or in response to a condition beingdetected that may have an impact on the factors (e.g., a certain numberof accesses to the flash cache memory 106).

In another embodiment, the smallest value of the maximum data size isgreater than zero.

In a further embodiment, the above throttling thresholds (maxfactor andmaximum data sizes) are chosen differently for data that is written(modified) vs. data that is read (unmodified). It may be advantageous tothrottle the unmodified data first or more aggressively than themodified data, as throttling the modified data has a direct negativeimpact on overall performance.

FIG. 4 is a process flow for throttling a rate at which data is writtento a flash cache based on a prefetch window size in accordance with anembodiment. In an embodiment, the process flow is implemented by logicin the storage controller 102.

The process depicted in FIG. 4 limits the amount of prefetchingperformed for data stored to the flash cache memory 106 on random reads.In an embodiment, prefetching waits for a read miss to occur, and thenfetches into the cache, in addition to the requested blocks of data, oneor more additional blocks that are spatially adjacent to the requestedblock. The number of additional blocks (or the size of the additionalblocks) is referred to herein as the prefetch window. In anotherembodiment, a variable block size is utilized (with a size equal to theprefetch window size), with larger cache blocks than originallyrequested fetched into the cache.

In an embodiment of the flash based cache described herein, data isfetched to the DRAM cache memory 104 first, and then from the DRAM cachememory 104 the data is sent to the flash cache memory 106. In theprocess flow shown in FIG. 4, the amount of prefetched data that is sentto the flash cache memory 106 is throttled. In the embodiment shown inFIG. 4, this is done by decoupling the size of the DRAM cache memoryprefetch window (or prefetch block size) from that of the flash cachememory prefetch window (or prefetch block size).

Referring to FIG. 4, at block 402, the maximum value of the TTF and theLTF is assigned to the variable “maxfactor.” In an embodiment, theprocess flow in FIG. 4 is called from the overall process flow depictedin FIG. 2 where the TTF and the LTF have been calculated. In anotherembodiment, part of the processing in block 402 includes calculating theTTF and the LTF. At block 404, it is determined if the maxfactor is lessthan 0. If the maxfactor is less than 0, then processing continues atblock 406, where the flash cache memory prefetch window size is set tothe DRAM flash cache memory prefetch window size. Thus, no throttling isperformed based on prefetch window size.

If the maxfactor is not less than 0, processing continues at block 408wherein it is determined if the maxfactor is less than 0.95. If themaxfactor is less than 95, then processing continues at block 410 wherethe flash cache memory prefetch window size is set to 0. Thus, no datais prefetched for the flash cache memory 106. If the maxfactor is lessthan 0, processing continues at block 412, where the flash cache memoryprefetch size is set to be equal to (DRAM cache memory prefetch size)multiplied by (0.95-maxfactor). Thus, the prefetch window size variesdepending on the value of the maxfactor.

In other embodiments where access to the flash cache memory 106 isthrottled based on a prefetch window size, a different maxfactorthreshold is used to restrict prefetches to the flash cache memory 106.In other embodiments, a percentage different than 0.95 is used in thecalculation in block 412. In another embodiment, the processing depictedin FIG. 4 loops back up to block 402 after blocks 406, 410, and 412 torecalculate the TTF, LTF and maxfactor for each data access. In afurther embodiment, the TTF, LTF and maxfactor are recalculated on aperiodic basis or in response to a condition being detected that mayhave an impact on the factors (e.g., a certain number of accesses to theflash cache memory 106). In other embodiments, the above throttlingthresholds (maxfactor and flash prefetch window size) are chosendifferently for data that is written (modified) vs. data that is read(unmodified).

FIG. 5 is a process flow for throttling a rate at which data is writtento a flash cache based on a data usage in accordance with an embodiment.In an embodiment usage throttling is performed by randomly selecting asubset of the originally requested pages that reside in DRAM cachememory 104 and only sending the selected subset of the originallyrequested pages to the flash cache memory 106. The selection can be madeusing fixed probabilities (e.g., randomly select 50% of the pages tosend to the flash cache memory 106) or by using dynamic probabilities asdescribed below. Note that randomly dropping unmodified data does notlead to data inconsistency, as a copy of the data is always present onthe hard disk. For modified data, the data cannot be simply dropped, butis simply written to the disks in the case when the throttling logicindicates that the flash cannot accept those modified data. Furthermore,an embodiment where pages are chosen probabilistically has the addedproperty that pages that are accessed multiple times will eventuallyenter the flash cache memory 106.

Referring to FIG. 5, at block 502, the maximum value of the TTF and theLTF is assigned to the variable “maxfactor.” In an embodiment, theprocess flow in FIG. 5 is called from the overall process flow depictedin FIG. 2 where the TTF and the LTF have been calculated. In anotherembodiment, part of the processing in block 502 includes calculating theTTF and the LTF. At block 504, a fraction of the pages read by the hostare randomly selected to be sent to the flash cache memory 106. In theembodiment shown in FIG. 5, the fraction varies based on the currentvalue of the maxfactor and is equal to (1-maxfactor).

In other embodiments, the fraction is set to a different value, such asthe absolute value of (0.95-maxfactor). In other embodiments, onefraction is selected for data that is written (modified) and anotherfraction is chosen for data that is read (unmodified).

The embodiments described above use the maximum value of the TTF and theLTF (i.e., the maxfactor) to determine the amount of throttling to applyto the flash cache memory 106. In other embodiments, just the TTF isused, just the LTF is used, or another factor is used to determine theamount of throttling to apply. In other embodiments, one group of one ormore factors and threshold values are used for one throttling process(e.g., data size throttling) and a different group of one or morefactors and threshold values is used for another throttling process(e.g., usage throttling). The groups may overlap or be mutuallyexclusive. Similarly, different groups of factors and threshold valuesmay be used for data that is read and data that is written.

Technical effects and benefits include the ability to use a flash devicefor cache memory in a memory system while supporting an expected usefullife of the flash device and without causing the flash device to becomea bottleneck in the memory system. This can lead to an increase inmemory system throughput.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

Further, as will be appreciated by one skilled in the art, aspects ofthe present invention may be embodied as a system, method, or computerprogram product. Accordingly, aspects of the present invention may takethe form of an entirely hardware embodiment, an entirely softwareembodiment (including firmware, resident software, micro-code, etc.) oran embodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain, or store a programfor use by or in connection with an instruction execution system,apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Smalltalk, C++ or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

Aspects of the present invention are described above with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

1. A method comprising: throttling accesses to a flash memory devicethat is part of a storage system that comprises the flash memory deviceand a second memory device, the throttling performed by logic externalto the flash memory device, the throttling comprising: calculating athrottling factor responsive to an estimated remaining lifespan of theflash memory device; determining whether the throttling factor exceeds athreshold; writing data to the flash memory device in response todetermining that the throttling factor does not exceed the threshold;and writing the data to the second memory device in response todetermining that the throttling factor exceeds the threshold.
 2. Themethod of claim 1, wherein the calculating the throttling factor isfurther responsive to a characteristic of the data.
 3. The method ofclaim 2, wherein the characteristic is a size of the data.
 4. The methodof claim 1, wherein the calculating the throttling factor is furtherresponsive to an estimated maximum throughput of the flash memorydevice.
 5. The method of claim 1, wherein the flash memory devicecomprises a cache memory.
 6. The method of claim 1, wherein the secondmemory device is a hard disk drive.
 7. A computer program productcomprising: a computer readable storage medium having computer readableprogram code embodied therewith, the computer readable program codecomprising computer readable program code configured for: throttlingaccesses to a flash memory device that is part of a storage system thatcomprises the flash memory device and a second memory device, thethrottling performed by logic external to the flash memory device, thethrottling comprising: calculating a throttling factor responsive to anestimated remaining lifespan of the flash memory device; determiningwhether the throttling factor exceeds a threshold; writing data to theflash memory device in response to determining that the throttlingfactor does not exceed the threshold; and writing the data to the secondmemory device in response to determining that the throttling factorexceeds the threshold.
 8. The computer program product of claim 7,wherein the calculating the throttling factor is further responsive to acharacteristic of the data.
 9. The computer program product of claim 8,wherein the characteristic is a size of the data.
 10. The computerprogram product of claim 7, wherein the calculating the throttlingfactor is further responsive to an estimated maximum throughput of theflash memory device.
 11. The computer program product of claim 7,wherein the flash memory device comprises a cache memory.
 12. Thecomputer program product of claim 7, wherein the second memory devicecomprises a hard disk drive.
 13. A method comprising: throttlingaccesses to a flash memory device that is part of a storage system thatcomprises the flash memory device and a second memory device, thethrottling performed by logic external to the flash memory device, thethrottling comprising: calculating a lifespan throttling factorresponsive to an estimated remaining lifespan of the flash memorydevice; calculating a throughput throttling factor responsive to anestimated maximum throughput of the flash memory device; calculating amaximum throttling factor responsive to the lifespan throttling factorand the throughput throttling factor; determining whether the maximumthrottling factor exceeds a threshold; and in response to determiningthat the maximum throttling factor exceeds the threshold, performing atleast one of data size throttling, prefetch window size throttling, andusage throttling; and writing data to at least one of the flash memorydevice and the second memory device in response to the throttling. 14.The method of claim 13, wherein the flash memory device comprises acache memory and the second memory device comprises a hard disk drive.15. The method of claim 13, wherein the performing the data sizethrottling is responsive to the maximum throttling factor and a size ofthe data, the performing the data size throttling directing the writingdata to the second memory device when the size of the data exceeds aselected size, wherein the performing the data size throttling comprisescalculating the selected size responsive to the maximum throttlingfactor.
 16. The method of claim 13, wherein the performing the prefetchwindow size throttling is responsive to the maximum throttling factorand comprises setting a size of a prefetch window when writing the datato the flash memory device.
 17. The method of claim 13, wherein theperforming the usage throttling comprises performing a random selectionof a fraction of received data to be written to the flash memory device,the fraction responsive to the maximum throttling factor.
 18. A computerprogram product comprising: a computer readable storage medium havingcomputer readable program code embodied therewith, the computer readableprogram code comprising computer readable program code configured for:throttling accesses to a flash memory device that is part of a storagesystem that comprises the flash memory device and a second memorydevice, the throttling performed by logic external to the flash memorydevice, the throttling comprising: calculating a lifespan throttlingfactor responsive to an estimated remaining lifespan of the flash memorydevice; calculating a throughput throttling factor responsive to anestimated maximum throughput of the flash memory device; calculating amaximum throttling factor responsive to the lifespan throttling factorand the throughput throttling factor; determining whether the maximumthrottling factor exceeds a threshold; and in response to determiningthat the maximum throttling factor exceeds the threshold, performing atleast one of data size throttling, prefetch window size throttling, andusage throttling; and writing data to at least one of the flash memorydevice and the second memory device in response to the throttling. 19.The computer program product of claim 18, wherein: the performing thedata size throttling is responsive to the maximum throttling factor anda size of the data, the performing the data size throttling directingthe writing data to the second memory device when the size of the dataexceeds a selected size, wherein the performing the data size throttlingcomprises calculating the selected size responsive to the maximumthrottling factor; the performing the prefetch window size throttling isresponsive to the maximum throttling factor and comprises setting a sizeof a prefetch window when writing the data to the flash memory device;and the performing the usage throttling comprises performing a randomselection of a fraction of received data to be written to the flashmemory device, the fraction responsive to the maximum throttling factor.